Generally speaking, an electronic fuel injector is installed at each cylinder in a branch portion of an intake manifold or at an intake port of an internal combustion engine. An apparatus for providing assist air-induced atomization of fuel is well known.
In such an apparatus, a portion of the intake air is guided along an assist air passage from the upstream side of a throttle valve, which is supplied in proximity to the injection hole of the fuel injector. Mixture of fuel with the assist air obtained thereby can improve flammability and emission by atomizing the injected fuel. But, when the assist air passage is blocked or when an assist air shift valve (AAI/V) that is installed in the assist air passage fails, the supply of assist air cannot be controlled. This detrimentally affects performance, fuel economy, and emission.
Japanese Published Unexamined Patent Application No. Heisei 3-217639, published on Sep. 25, 1991, exemplifies a previously proposed diagnostic apparatus for supplying assist air in an internal combustion engine. In that application, when the AAI/V changes from off to on and the engine rotation speed reaches its peak, the occurrence of an abnormal condition of the system is diagnosed. However, since the detection of the abnormal condition is based on only one diagnostic result, it may cause a wrong diagnosis when the engine conditions (e.g., driving condition, varying time) for diagnosis varies greatly. Also, in the case of diagnosis based on the peak engine rotation speed, the rising rate of the engine rotation speed is changed by environmental conditions, such as altitude and intake air temperature, as well as by engine conditions, variations in engine parts, and variations in engines themselves. As a result, the precision of the resulting diagnosis may be adversely affected.